Apparatus for burning or calcining materials



Marh 175 1927.,

lgg H. M. ROBERTSON APPARATUS FOR BURNING 0R CALCINING MATERIALS F11 .ed June' 20. 1,925 7 sheets-sheet .A AAVALVAFWAIFIAFEAL 1 IN VEN TOR W1. WMM/@7L l? ron/vn Lmzz arch 15, 1927.

H. M. ROBERTSON APPARATUS FOR BURNING OR CALCINING MATERIALS Filed June 20. 1925 '7 Sheets-Sh't 2 I N vf/v To@ 77( (441 J TOR/VE Y.

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Tron/V57 March 15, 1927, H. M. ROBERTSON 621222 APPARATUS FOR BURNING OR CALCINING MATERIALS Filed June 20, 1925 v sheets-sheet v' ffl. @y mm@ Patented Mar.- 15,( 1927.

UNITED STATES HARRY M. ROBERTSQN, F LAKEWOOD, OHIO.

APPARATUS 'AIEOR BURNING OR CALCINING MATERIALS.

applicati@ mea June 2o, i625, serial Na. 38,589.I

^-This invention relates to an apparatus for and process of burning or calcining materials, particularly-materials in rock form,

such as limestone, shale and the like. The apparatus is of the continuous kiln type.

One object of the invention is to provide an improved apparatus for burning or calcining materials, whereby uniformity of product is insured, greater output results and operating expenses' in fuel and labor are decreased.

A further object of the invention is to produce an improved apparatus wherein limestone may be calcined by contact with the products of combustion Aand complete burning effected so that picking of the calcined material is eliminated.

Another object of the invention is to provide an improved process of calcining limestone capable of producing lime ofL high quality in a rapid manner.

Another object of the invention is toprovide an improved tunnel kiln of simple construction Other objects of the invention will be apparent to those skilled in the art to. which my invention relates from the following de-` scription taken in connection with the accompanying drawings, wherein Fig. 1 is a plan view bf an apparatus ernv bodying my invention and capable of use in carrying out my improved process.

Fig. 2 is a longitudinal section through th apparatus.

Fig. 3 is a fragmentary view, enlarged, showing the preferred manner of introducing the materials into the apparatus and `conveying them therethrough.

Figs. 4, 5 and 6 are fragmentary enlarged sections on the lines 44, 5 5 and 6 6, respectively, of Fig. 1.

Figs. 7 and 8 are enlarged transverse-sections on the lines 7 7 and 8 8, respectively,

a holder for the material.

Fig. 12 is a perspective view of a truck (the wheels therefor lbeing omitted) having a holder of slightly different form from that shown in Fig. .11.

Fig. 12a is alongitudinal sectional view of the truck and holder shown in Fig. 12,-on4

the iline 12 12a thereof.

Fig. 13 is a perspectiveview of a truck (the wheels therefor being omitted) having ya holder slightly different in construction' from that .shown in Figs. 11 and 12.l

Fig. 14 is a fragmentary view, artly in section, of the discharge end of t e apparatus showing the automatic means for discharging the materials. l

Fig. 15 is a section on the line 15 15 of Fig. 14.

My apparatus may be used for burnin various kinds of materials, including sha e and limestone, but I have particularly adapted it for the burning or calcining of the latter. In my apparatus and according to my process the limestone rocks are subjected'to heat at a predetermined temperature for a predetermined period of timev without contact or mixture with the fuel, so that .lime devoid of ash and other products is obtained. In carrying out my process, I use an apparatus of the continuous kiln ty e,

whereby the introduction of the material into the kiln, its treatment and the discharge .thereof may be effected readily and substantially continuously.

-1 indicates as an entirety a kiln having a burning section 2, a preheating section 3 and a cooling section 4; and by reference the preheating section is provide with an inlet section 5 and the cooling section 4 is provided with a supplemental or auxiliary cooling section 6, all of these sections being in line with each other, whereby the :material is passed through a zone 3a of gradually rising temperature, then through a zone 2B having a fixed, predetermined temperature to effect the burning of .the material, then through a zone 4a, whereby cooling is permitted; then through a kzone 6a wherea further cooling is provided for.

The kiln is built on a suitable bed 1 preferably formed of cement. The outer walls of the kiln 1 may be built-of bricks- 7 or other materials. These walls may be braced by studs 8, tied together by rods .9, the studs being spaced any desired distances and fixed at their lower ends in the bed` la. The walls 7 are lined with insulation, as shown at 10, to properly insulate the inner walls k11, which are preferably formed of fire bricks.- The walls ofthe inlet section 5 and supplemental cooling section 6 may be formed from cement. All of the walls that widenedrat its lower portion to form benches.

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:dare formedifrom cement-mayl be .reinforced is introduced substantially centrally of the'- kiln and at the point of maximum temperafv in any manner desired.` The inner wall 1I is 12 which extend from end to'end of the kiln 1 and form between thema 'space for'lthe conveying means for the limestone rock, such ture. The zones 3, 2 and 4 are covered by roofs 3a', 2a', 4R', respectivel each of the roofs 31, 2a', preferably eac comprising an arch of-'suitable fire bricks 22 covered conveying means in thev resent instancelwith insulation 23 andthe roofeta compriscomprising trucks indicate as -an entirety at 13, to which reference willl'ater be `mada f 4l0Aswill later appear, the trucks 13` fit -between the benches and run on trackslfi and cooperate with the benches to form above the latter the zones 3, 2, and 4, whereby the heat and roducts of combustion and carbon dioxi e and other vapors maybe `con zone 3*.

trolled and readil discharged or exhausted( `from the kiln an the products of combustion utilized to the best advantage in preheating the material as it passesthrough the The benches 12`alsog-serve as' supports for baflies 14, '15, 16, in the zones 3, 2l and 4, respectively, the purpose of which -will later be set forth. 'The benches 12 may be variously constructedfrom fire bricks;

' for example, each may be made to forman 39'i dicated as` an entirety interior space that is filled with cement, as shownatlif; while portionsof the benches 12'may'be constructed otherwise to provide forfthe primary combustion chambers, in-

at 18 or foroutlet L conduits19 (see Figs. 11 and 7). The opposingf side fac s 'of the benches 12 are provided with- .ledges 20 which cooperate with the sides of thetrucks 13 to prevent the heat and) products of combustion from flowing down f v""ad"acen"t 4each lbench side `face below the belowgthe truckbodies 13a and causing damage' to lthetruck wheels, their axles and axle bearings. "5 To'- maintain the space below the ledges 20 relatively cool, I provide piping adjacent ledge 20 (being mounted on thc side face in any desired manner) and circulate through such piping cold air by means of a suitable blower (not shown) connected to the inlet ends of the pipes 21.l As will .be understoood from Figs. 1, 2 and 5,there are two sets of pipes 21 on each bench side face, one set extending from the inner end :of the preheating section 3 to its outer end yand the other set extending from the inlet end of the burning section 2 to or a point -adjacent the discharge end of the cooling `zone 4a. The outer and inner ends of the vpipes 21 are .bent -outwardly and project through the walls of the kiln 1. The inner ends of the sets of pipes are connected by a pipe 21 with a suitable blower (not shown), 4which 'forces cold air through them, the radiation from the pipes serving to maintain the space between the benches 12 and below the ledges 20 and truck bodies relatively cool 4'for the purpose already set forth. By this arrangement of the pipes and their connectionwith the cold air supply, the cold lair in g. an. arch of suitable iire brick 23.

The burning or calcining of the limestone is effected While'lpassing from one end to the other end of the furnace section 2 or. in passing through the burning zone 2a, wherein a predetern'lined temperature, preferably of 1,700 to.j 42,400 degrees lFahrenheit dependent upon the quality and nature of the limestone is maintained, although a higher or lower temperature may be maintained if desired, I having found that Within certain limits, a relatively'high temperature is de sired to burn out the impurities in the material being treated to produce what is commonly known as White lime. The heating means comprises a pair 'off combustion chambers 24 extending fromv'end t'oend of the furnace section 2 preferably on or above the level of the benches 12 at each side of the burning zone 2n (see Fig. 9), the primarycombustion chambers 18 (already referred to) and air supply conduits 25, 26,

arranged to supply heated air to the combustion chambers 24, as will later appear..

wall 18a and aA rear wall 18b that is upwardly inclined, these Walls forming an opening 18 at their inner portions that leads into the bottom` of the adjacent com-y bustion chamber 24'. `By' preference the walls 18at and 18b of each primary combustion chamber are 'cut away to form a' groove or space 18c of any desired Width leading inwardly through the wall 7 and the walls 18a and 18h are provided with a. lining 181,`

preferably formed from slabs of suitable refractory material, to protect the Walls of the benches 12. As shown in Fig. 9, the

lining 18d extends over and along the groove 18c and cooperates with its Walls to form the air supply conduit By this arrangement the air in the lconduit 25 is heated bv the burning fuel in the chamber 18 and deby provision of a grate they may be. utilized 4 to burn coal as the fuel. 26vcomprises a manifold 27, which opens Each air supply A into the adjacent combustion chamber 24 preferably at that end adjacent the disch'arge end of the burning section. 2. The air supplied by each manifold 27 .1s heated in the following manner: 28 indicates a conduit or pipe incorporated in each side lwall of the cooling section f4, preferabl adjacent its discharge end. The pipe 28` as an inends to the adjacent manifoldy 27. The air.,

`in the pipes 30 is heated by the `heat radiating from the material passing through' the cooling( zone 4, the transfer \Of the heat from the materialtothe air fin the pipes serving to gradually-'cool tl1e 'material and to heat the air, so'that the` latter may be used for combustion. purposes in the com-s bustion chambers" 2,4 to` supplement the heated air supplied by thev conduits 25. `T he pipes 30 ar'emounted in 'openings for/med in the spaced piers or baffle members y16,- which f are 'supported-on -and extend 1 from the' ben'ehes12 vto the arch 43'] .The piers 16 ltend to cause the heat to radiate i'nwardly and outwardly relative to the? pipes ',30fdue`to a draft which is set up within the -kiln 1,y as Vtwill be later described. The i pipesv30 may be formed from fire clay or! between them, and'thus prevent these cleother suitable. lire resistingmaterial. By

n akingfthemof rectangular shape, maximufmarea of exposed pipe walls is-provided between the inner and outer edges of the piers 16, or at either 'side of the space required for-the passage of the material.

The combustion chambers 24 .extend from end'to end of the burning section 2, each being disposed above the adjacent bench so as to be on an approximate level'with the material passing through the zone 2a. Each chamber 24 has 'a rear refractory wall 32 1 to vprotect the inner` wall 4l1, and a front comprises slabs of suitable refractory material supported on the upper ends of the walls 32 and 33, as shown in Fig. 9. The

4 baflles or partitions 15 extend from the tops 35 to the arch 22 have their front edges in a plane coincident with the front or inner face of the wall 33. The baffles 15 are preferably arranged one opposite the other and uniformly spaced from end to end of the zone 2a (see Fig. 5) and primarily serve to prevent the products of combustion and the heat generated thereby from finding a straight path of exit through the zone 2a;

on the other hand, the batlles .15 serve to cause these elements and other gases or vapors to flow inwardly and outwardly Vin a successive manner as they are drawn through the zone 2"L to the zone 3a by the induced suction, to which reference will alater be made; accordingly it will be seen that the products of combustion and other heatedgases and vapors are caused to circulate-into andthrough or between thermaterial and out again.

The heated air and gases, consumed lproducts of combustion, carbon dioxide and other vapors given Off from the material -a's it passes through the zone 2 flow into `and through the preheating zone *3a and Contact with the material being moved forwardly therein, lthe' effect of which is to dry out all moisture in the material and gradually heat it up to approximately the temperature of the' zone 2a. To more effectively cause these heated elements to penetrate the material as vit moves. through the zonef3, I provide therein a plurality of pairs of piers or lbafl'le elements 14, (already referred to), disposed in spaced relation preferably equal distances from each other. As understood Y from Fig. A1, the baffle elements 14 constituting each pair are positioned lopposite each other on the benches 12, the effect of which arrangement is to cause the heated elements to flow inwardly. around these baflle elements and then outwardly into the spaces ments from forming into a current and flowing in'a path leading directly through the zone 3a' these baflies therefore insure the circulation of lthese elements into and through the material or the spaces between the rocks lconstituting it. l

A38 indicates a draftfinducing means connected withthe. zone 3u and serving to set up a suction effect therein and thus draw all of the hea-tedfair and gases, products of combustion or gases resulting therefrom, and vapors from the material through the Zone lid 2a into the zone 3a and through the latter whereby they are discharged either into the open, or ito some suitable apparatus where it is desired to re-elaim, as by-products, any of the materials combined or mixed with the limestone or other material that is being ealcined. The draft inducing means is regulated to insure flow lof all the heat and gases and products of combustion and prevent any portion thereof entering the cooling zone 4a.

l 39 indicates as an entirety mechanism for regulating the dlscharge of the heated air' and gases, vapors and consumed products of combustion from the zone 3a. draftv inducing means and the regulating mechanism are in combined or associated As the relation, I will describe them as 'followsz one-iside of the preheating section and lleading toa suitable point of discharge. Interposed in this conduit is ay suitable suction "device, such as a power driven fan (not shown) having suflicient` `suction effect to induce the 'maximum draft in' thel zones 3a and 2, 41 indicates a conduit mounted on the roof 3 and extending longitudinally and Vcentrally thereof soas to receive the heated ingredients and elements of the character referred to through one or more ports 42 formed in the roof 3a. The ports 42 may damper-s, the discharge of the heated ingredients at one or several points can be effected. The conduit 41 preferably comprises an inverted hollow member which sets into aslot or opening formed in the roof section 23 as shown in Fig. 4. 43 indicates a pipe connecting the conduit41 with the conduit 40. As will be understood from Figs. 1 and 4, the conduit 41 extends downwardly to a point below the bed la, where it is connected to a transverse pipe 44, which in turn is connected by risers with the chambers or outlet conduits 19V (see Figs. 4 and 7). The chambers -19 preferably exi tend longitudinally of the benches 12 from the inlet end of the zone 3aL to a point below 'the third pair of baffle elements 14. Each`lv 'number' of.' advantages result in the use of the balies 14 above theadjacent conduit 19 has a stepped rear end wall 45 for a pur' pose which will later appear. 46 indicatesf a plurality of vertical pipes or condults .formed in each 'of the baffle elements just referredA to and extending from the adjacent` chamber-,19 upwardly and each pipe 46 is connected to one or` more inlet pipes or openings 47 leading inwardly from the end wall whereby the conduit 19 receives the heated` ingredients from points at different levels within the zone 3*. 48 indicates dampers for closing the 'pipes 46, each damper lpreferably comprising a plate slidably fit' ting a guide Way 49 formed in the adjacent bench 12and walls 11 and 7-see Fig. 7. By pulling one or more of the dampers out, the flow of the gases can be, controlled through any one or more of the pipes 46 and at a high orlow level in the zone 3a, as desired. By providing each of the baffle elements" with a larger number of pipes 46 or controlling the openings into the pipes v 47 the discharge of the heated gasescan be taken ofi' at intermediate levels.

From the foregoing description it will .be seen that I provide for the discharge of the heated gases either at the top of the zone 3a or .at dilferent levels in the lzone, and

zone through either of these discharge o en.-

ings. Accordingly, ,the suction effect o thel fan can be regulated dependent upon the conditionsexisting within the kilnV or the nature of the material or speed with which it becomes heatedv after it enters the zone-3a. i rEhe .supplemental Icooling section 6 may be provided with supplemental means 50 for cooling the material rapidly, since lthe material after passing through the cooling zone4a has cooled sufficiently to permit the application of an sortof cooling without Idanger of affecting the material'. or the holder 13 carrying it. The supplement cool'- mg means 50 comprises a pair longitudinally arranged upper and lower manifolds 5l connected by a plurality of pipes `52, arv ranged adjacent each side wall of the Section 6. The upper manifolds 51 are connected by pipes 51a to a pipe 53 which extends .upwardly through the roof ofthe section 6. 'A suitable blower (not shown) is interposed in the pipe 53, the purpose of` which is to blow or suck cold air through the manifolds 51 and pipes 52, openings being formed in the lower manifolds to' permit of this operation (see Fig. 6).

The limestone may be loaded onto trucks carrying holders of the type shown generally at 13 in Figs. 12 and l2a and the trucks moved successively throughthe kiln 1, preferably in abutting relation; but I prefer to use trucks having holders of the type shown at 13 in Fig. 11 or of the construction shown at 13 in Fig. 13, since a thereof, to wit, (1) the holders can be continuously loaded without waste of material;

(2) the holders, by reason of their continuity while being moved through the kiln, have an increased carrying capacity and provide for "the movementof a continuousperature without damage or deterioration of any sort. As shown, the trucks carrying the holders are moved into filling position, as shown in Fig. 3, preferably below a loading hopper 54, then they are moved into an/d through the zones 3, 2a, 4a, successively,

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thenthey are moved past a discharging or unloading ymechanism indicated as an 4entirety at 55; then the trucks arev returned by a. return track, (not shown) to position ready to lbe moved to a point below the loading hopper 54.' rIhe movement of the trucks may be continuous or intermittent,

3as desired, power being applied to the first ening ribs and a flange 56b extending along its sides and endssee Fig. 121) and bearing boxes 57 for axles 58 which carry the wheels 59, the peripheries of which are shaped to engage the tracks 14', 60 indicates a base portion supported on the frame 56 and formed of refractory blocks, such as bricks 61, as shown in Figs.v 11, 12 and `12a or tile 62, as shown in Fig. 13. The bricks or blocks rest on the frame and fit within the flange 56h. The upper surface of the base portion 60 is shaped to form inclined bases 60, 60, extending fronrthev longitudinal side edges of the basef'portion downwardly to a longitudinal line extending centrally thereof. The purpose of this construction will later appear. A13-6 3 the end walls of the base portion'GO arev `provided with transversely extending ribs arranged to close the spaces between the trucks and thus i prevent heat from radiating downwardly. As shown in Fig. 3,1the ribs on lthe front and rear end walls of eac-h truck are olf-set one relative to the other so that the ribs' on adjoining trucks will overlap each other for this purpose.'`

The holder 13 comprises a bottom 64, side l walls 65 and supporting members 66 all of which are constructed from blocks 67 and slabs 68 each formed from suitable fire clay capable of resisting the highv temperature! required in the kiln l in burning the limestone or other material and exposure to the` air at normaltemperature without danger of breakage. The blocks 67 preferably eX- tend transversely of the base portion G0, being arranged inrows each row vspaced from the other row'to permit the heat to circulate beneath thejbottom V64 and side Walls 65; also, the blocks in each row constitute a series so that their lower surfaces may engage the incline basesl g1 and their upper surfaces support the slabs 68 in themanner shown. The side walls 68 are inclined inwardly and downwardly, thus serving to suiiiciently to maintain a maximum load without affecting the accessibility of the heat and products of combustion to the mate- 68 forming the side walls 65 are inclined rial or free escapeiherefrom ofthe 'vapors' given off incident to the burning action. It wil'llbe seen from Figs. 3 and 11 that'the bottom 64 and side walls 65 extend from end to end of the truck body 56 or vbase portion 'I0 60, so that they substantially abut the corresponding Walls of the adjacent trucks. As a result, the loading of the trucks may be. carried on continuously and the holders may co-operatcone with another to form a continuously moving stream of material which is continually moving into and through the kiln, as will '.be understood from Fig. 3/

The holder may be modified so that the outermost slabs thereof will be but Slight] inclined, as shown at 69 in Fi 13, in whic form of construction I pre er to ,provide flange sections 70'extending along the side edges of the slabs 69. As will be understood, the flangesections are formed of the same material as the other sections of the holderg also, that the blocks 67 are shaped to support the slabs 68 and 69 in the position shown. Fig. 13 also shows. a further modification which may be used in connection with eitherform of holder, in that the base portion is constructed of fire-clay blocks ina seat .for the adjacent slabs 68a. By fitting the base portion 60 of the holders within the flange 56D, the latter will lie beyond the refractory base portion and thus engage each other to effect movement of the trucks through the kiln 1 without contact with the refractory walls. Danger of breakage of these walls is'therefore eliminated.

In my proces'sof burning-limestone, the

material is first introduced into and passed through a zone where the temperature is gradually increased from approximately atmospheric temperature to the temperature at which the burning is effected, then the material enters and passes through a zone of a predetermined temperature so that for a lpredetermined periodthe material is subjected tothe desired temperature t`o insure complete burning, and finally the material passes into and through a cooling zone where its heat is utilized to heat air which is supplied to the combustion chamber. During the passage of the material through these Zones', it is continuously subjected to a draft induced at or near the entrance' end of the kiln, the effect of which is to carry off the vapors resulting from the burning of the material to draw the products of combustion and heat'rearwardly and' through tlie zone 2* to prevent it passlng into the cooling zone 4'@ and to direct it into and u on the material being moved forwardly in t e zone lf/From the foregoing description it will be sen that in my form of construction and in carrying out my process, I provide for a continuous-sup ly of materialand movement thereof throng the kiln, and finally discharge it from the holders outside of and beyond the exit end'ofthe kiln, preferably as shown in Figs. 14 and 15. The holders are preferably loaded from a hopper havin a2 suitable chute/54 controlled 15' by a valve not shown). The bottom or delivery end of the chute 54'* may be so positioned that it will level over the material discharged into the" holders at such height @sto prevent overloading thereof;

.a 'collecting hopper countersunk in the round below the tracks 14 and bed 1". This o per may have its Wall built of cement an its lowermostl portion may be shaped to form a chamber 73 through which an end.` fless conveyor 74 runs, the walls ofthe chamber having uide members 75 to direct the burnt material into the buckets. The conveyor buckets may carry rollers 74, engagving suitable tracks 74h. The conveyor 74 leads to a suitable discharge place or station.

76 indicates frames disposed on opposite sides of the tracks 14', each frame pre erably comprising u rights 77 and connecting members 78, 79 in icates the discharging elements ,comprising a series of scraping members extending between the frames 76 and ar-x ranged with their lowerl free edges at different levels, the first being arranged to scrape off a portion of the material as a holder 13 is moved below and past the scraping member, the next being arranged to scrape off a further portion of the material as the movement of the holder continues and the last scraping y member being arranged to scrape closeto the bottom of the holder and thus effect discharge of all of the material.'

It will be obvious that the holders could remain stationary and the scraping members moved relative thereto, in which event 'the .uprights 77 could be mounted on wheels, but

I prefer to continue the tracks 14 outwardly beyond the kiln section 6 and hopper 72 and to move the truck thereover as s own in .55 Fig. 14.` In moving the truck and its holder lOf the discharging'means 55 `7 2 indicatesy from the succeeding truck to be regulated according to the chemical char. 70

acteristics of the limestone. As will be'noted yvery accurate control' of the tempera-ture' may be made, lso lthat a better quality ora higher grade of lime canl be produced than`v with apparatus now in use. Theperiod off burning depends v upon the length of @the heating section, which will be determined by .the available limestone" at the place-`of'-erec.`

tion ofthe kiln. Since the material' is movedv through a burnin zone andonlythe` gases 80 of the products o combustion J come-in ontact therewith, and the temperature of these gasesmay be controlled by regulation'of the fuel and air, it will be seen that relatively hi h eiiiciency is obtained without dangero overheating. j

As my apparatus maintains theV limestone and fuel. separate, the lime -may be used fory chemical pur oses and furthermore aclimestone which reaks up under theiniiuence of heat may be calcined.

In my construction,.the material iscon``l lveyed through the kiln and calcined` without contact with the walls thereof. Accordingly no abrasive action takes place. Asa result, the inner exposed walls of the 'kiln maybe constructed from variousv materials capable of withstanding the various temperatures existing in the `pre-heating, burning and cooling zones without regard to their lhardness or to the chemical characteristics of the limestone to be calcined or the amount of impurities contained in it. Not only does this constructioneconomize the cost, but simplifies the building of the kiln and eliminates the renewal ofvand repairs to the lining required in other types of kilns.

From the foregoin description it will be seen that I provide or complete regulation of'the'induced draft so that (l) the burning conditions are properly controlled to prevent .the heat and gases from entering the coolingk zone to effect their iow toward the exhaust gort or ports; and (2) the temperature in ifferent portions of the pre-heating zone 3 1.15.

may be changed at will to meet varying con- Vditions.

I have found that in a continuous tunnel kiln of the construction hereinv disclosed,

very material reduction in fuel and labor may be attained. Furthermore, the'apparatus is capable of calcining limestone rock .material that is relatively small or fine in Laarzen l themselves Without departing from the spirit and scope thereof. My disclosures'and description herein are purelyillustrative and rare not intended to be in any sense limiting.

.What I claim is: t

1. In a tunnel kiln\foricalcining material, the combination of a pre-heating section and a burning section, the burning section comprising an elongated central portion through which material is conveyed and ay plurality: of secondary combustion chambers disposed in each side Wall of the burning section and opening into the saidcentral portion from -end to end thereof, a plurality of primary combustionv chambersqdisposed 'Within the side Walls of'said burningsection and having exit openings leading ii'ito the lower portion of .said secondary chambers, air inlets for 'the latter chambers, and means for con- 4veying material throughl said sections.

2. In av tunnel kiln,`the combination of a pre-heating section and a burning section,y

the burning section comprising an elongated central portion through which material is j conveyed and a plurality of secondary combustion chambers disposed in each side wall of the burning section and opening into said central portion from end to end thereof, a plurality of primary combustion chambers disposed Within the ,side Walls of said burning section and having exit openings leading into the lower portion of said secondary.

' combustion chambers, means for supplying air to said secondary combustion chambers, holders for conveying material through said sections, and means at the discharge end of said kiln for discharging the material from said holders.

3. -In a tunnel kiln for calcining material, the combination of a pre-heating section, a burning section and a cooling section, the burning section comprising an elongated Central portion through whichv material is conveyed and a plurality of secondary combustion chambers disposed in each side Wall v'of the burning section and opening into the said central portionA from end to end thereof, a plurality of primary combustion chambers disposed within the side Walls of said burning section and having exit openings leading into the lower portion of said secondary combustion chambers, air conduits extending from the discharge end of said cooling section inwardly to and connected with the secondary combustion chamber adjacent the discharge end of said burning section, and means for conveying rock material through said sections.

4. In a tunnel kiln for calcining material,

the combination of a pre-heating section and a burning section, the burning section comprising an elongated central portion through which material is conveyed and a plurality of secondary combustion chambers disposed in each side wall of the burning section and opening into the said central portion -from. end to end thereof; a pluralit of primary combustion .chambers dispose vwithin the sidewalls of said burning section and having-i4 exits for comnrunicating With said secondary l combustion chambers, means for supplying air to said secondary combustion chambers; y means for conveying material through said' sectlons, and means spaced along said pre-- l heat-ing section `adjacent its inlet end -for regulating-the exhaust o'fgasesl and vapors from said pre-heating'section.

5. In a tunnel kiln for calcining material,` the combination of a pre-heating section and a burning section, the burning Asection cmpri'sing an elongated central portion ythrough which 4material Yis conveyed and a plurality ofsecondaryi combustion chambers disposed '1n each side Wall of the burning section and p opening into the said central portion from lend to end thereof; a plurality of primary, combustion chambers incorporated -in the` side Walls of said burning vsection and/having inlets communicating with saidVt secondary combustion chambers, means fori'sup- 90 plying air to said combustion chambers,-" means for conveying material through said sections, 'and means spaced along said preheating section adjacent its inlet end for regulating the exhaust of gases and vapors from said pre-heating section, said regulating means including' a plurality of valved` outlets in the upper portion of said section.

6. In a tunnel kiln for calcining material, the combination of a pre-heating section and a burning section, the burning section comprising an elongated central portion through Which material is conveyed and a plurality of se :ondary combustion chambers disposed in each side Wall of the burning section and opening into the saidcentral' portion from end to end thereof; a plurality of primary combustion chambers incorporated in the side Walls of said burning section and having inlets communicating With lsaid secondary combustion chambers, means for supplying air to said secondary combustion chambers, and means spaced along said pre-heating section adjacent its inlet end for regulating the exhaust of gases and vapors from said pre-heating section, said regulatin means including a plurality of valve outlets having intake openings disposed at different levels in said pre-heating Jsaid burning section and having inlets communicatingwith said secondary combustion chambers,.means for su plying air to Jsaid secondary 4combustion c alnbers, means for conveying material through said sections, and means spaced along said pre-heating section adjacent its inlet end for regulating the exhaust of gases and vapors from said pre-heating section, said regulating means comprisingv a plurality of valved outlets-1n the upper portion of said pre-heating section and-a plurality of valyed outlets having intake o enings disposed -at different levels below t 1e upper portion of said prehating section.

8. In a tunnel kiln for calcining material,

the combination of a pre-heating section and a burning section, means for conveying material through'said sections, ythe burning section comprislng an elongated central-porburned, eachfsaid primary combustion chamber comprising'a chamber having side Walls and a refractory bottom extending rearward to direct the flame `into the adjacent secon ary combustion chamber, and a conduit communicating with the atmosphere andfextending below said'v refractory bottom of each primary lcombustion chamber andg at'its inner' end with thel communicatin lower portion of the adjacent *combustion-l chamber, "wherebythe heat from the fuel :burning in said primary combustion cham-` ber heats the air in .said conduit vsupplied thereby vfor combustion purposes in said l secondaryl combustion chamber.

9. Inapparatus of the class described, the combinationAv of a tunnel kiln for calcining materials having an inlet andoutlet, trucks Carrying holders for material having open endsfnreans for moving said trucks through said kiln, and discharge means disposed beyondthe outlet end o said kiln and in line therewith and acting on-the material due togrelative movement between said means nd each truck to scrape the material thererom. i

10. Anap aratus as claimed in claim 9 in which the ischarging' means com rises a scraping device disposed in the pat ofi-the material as the truck is moved-outwardly from said outlet.v

11. An apparatus which the discharge means comprising a scraping device disposed at right angles to the movement of the truck and in the path of movement of the material as the truck moves outwardly from said outlet.

12. An apparatus as claimed in claim 9 in which the discharging means comprises a plurality of scraping devices dis osed at right angles to the movement of t e truck outwardly from said outlet, said devices having their lower edges in stepped relation downwardly from the first device to theilast device.

v 13. In apparatus of the class described, the combination of a tunnel kiln for' cal-l cining materials having an inlet and an outlet, trucks carrying holders having bottom and side walls, means for moving said trucks through said kiln, and a plurality of scraping devices arranged to engage successive portions of the material and discharge it rearwardly from said'holder as the truck moves outwardly from said outlet.

14. vAn apparatus asclaimed in claim 13 in which the rearmost scraping device is shaped to fit the bottom and side walls of said holders.

. In testimony whereof, have hereunto subscribed lmy, name; f HARRY M. ROBERTSON.

as claimed inclaim 9 in- 

